The present invention relates generally to a flow cell apparatus and, in particular, relates to such an apparatus for equilibrating the temperature of a fluid passing therethrough.
Sample flow cells are available in a variety of shapes, sizes and configurations. In addition, heat exchangers for use therewith are also known. The primary purpose of previous heat exchangers is merely to remove thermal energy from the flowing fluid containing the sample before it reaches the sample cell. Very often such a heat exchanger functions by transferring thermal energy to a working fluid, which can, of course, include a gas and which subsequently carries the transferred thermal energy away from its source. Alternatively, the heat may be transferred to the main structure of the instrument containing the sample flow cell. If this structure is relatively large and the heat carried by the flowing sample is relatively small, the heat may be dissipated to the environment. However, if the heat flow is relatively large, the entire instrument may be disturbed and result in inaccuracies.
Further, the extra path length and extra volume of the heat exchange tubing through which the flowing sample must pass causes remixing of adjacent but separate components in the flowing sample. The extent of remixing depends on the volume and length of the tubing through which it must pass. In applications such as liquid chromatography for example, where the separation of components is achieved at significant cost in time and apparatus, the partial remixing of the components in the heat exchanger can significantly reduce the performance of the apparatus.
For these reasons, it is important to effect any desired heat exchange with a minimum of length, and consequently volume, of tubing.
However, in many applications, the simple removal of thermal energy from the flowing sample is not as important as is the ability to equilibrate the temperature of the flowing fluid with the device through which the fluid passes.
One device where such equilibration is highly desirable is the flow cell of a liquid chromatography system. In a liquid chromatography system, the eluate of the separating column may be required by the particular analysis to exit at a rather elevated temperature. The eluate is conveyed, via a short connecting conduit, to a measuring flow cell. Often, a measuring light beam is passed through the flow cell and the intensity reduction of the light beam during its passage through the cell is a measure of the absorbance of the fluid passing therethrough. Frequently, if the difference in temperature between the eluate of the column and the body of the flow cell is excessive, unacceptable noise results in the detection mechanism. Hence, it is often desirable to reduce this temperature difference while employing a very short length and very small volume of heat exchange tubing.
One way to minimize the temperature difference between the flowing sample and the flow cell is to thermally isolate the flow cell from the main structure of the instrument and exchange heat between the fluid and the flow cell before the fluid enters the measurement part of the cell. This results in the temperature of the flow cell changing until it nearly equals that of the incoming fluid. However, if the sample is much hotter or much colder than the main structure of the instrument, difficulties of a different kind can result. Some of the difficulties are, the thermal expansion of the cell relative to its mounts, thermal convection of air around the cell, the formation of condensation on the cell windows, etc.